Fuel cells are often selected for installation on vehicles because of their lower impact on the environment. To obtain an operating point for controlling such a fuel cell, the current-voltage characteristics, i.e. the I-V curve, are often used as output characteristics of the fuel cell. The intersection of this I-V curve and a required power curve is set as an operating point.
In a fuel cell I-V curve, the output voltage has a maximum value when the current is 0. As the state in which the current value is 0 corresponds to a state in which both output terminals of the fuel cell are opened, the voltage at this time is referred to as an Open Circuit Voltage (OCV). When the current is increased from this OCV state, the voltage value of the fuel cell abruptly decreases and is then kept at a fixed value, and a region in which the output voltage does not change significantly even with the increase in the current value continues. When the current value further increases beyond this region in which the output voltage of the fuel cell is stable, the voltage value then sharply drops from the fixed value to 0. As such, the I-V curve of the fuel cell exhibits non-linearity due to the electrochemical reaction of the fuel cell, in which the output voltage of the fuel cell sharply changes near the OCV and the region in which the current value is great. Because degradation of a catalyst which is one structural component of a fuel cell is accelerated when the fuel cell is operated near the OCV, setting of the operating point of the fuel cell is performed while avoiding the region near the OCV.
For example, Japanese Patent Publication JP 2007-5038 A (Patent Document 1) indicates that, in a fuel cell system, the output characteristics of a fuel cell are degraded due to a sintering phenomenon when the output voltage of the fuel cell changes over the oxidation-reduction potential in accordance with a fluctuation in the system required voltage. Accordingly, when the FC required power is less than the oxidation-reduction power, the FC output voltage is set to an OCV (open circuit voltage) such that power is not generated, whereas, when the FC required power is greater than the oxidation-reduction power, the FC output voltage is set to a voltage corresponding to the required power. Then, even when the FC required power subsequently decreases to fall below the oxidation-reduction power, the FC output voltage is set to the oxidation-reduction potential during a continued time period reaching the fitness value β, thereby continuing power generation. Once the fitness value β is exceeded, power generation is no longer necessary, and no power is generated. As described above, Patent Document 1 describes avoiding the FC output voltage from changing over the oxidation-reduction potential.                Patent Document 1: JP 2007-5038 A        